High-frequency furnace circuit



- l 1,642,198 Sept. 13s 1927- F GERTH ET AL HIGH FREQUENCY FURNACCCIRCUIT Filed Feb. 15, 1926 r Patented Sept. 13,-19727..

, UNITED STATES 4 1,642,198 lPATENT OFFICE.

FELIX GERTH AND WILHELM FISCHER, OF? BERLIN-TEMPELHOF, GERMANY, AS-SIGNORS TO C. LORENZ AKTIENGESELLSCHAFT, OF BERLIN-TEMPELHOF, GER- MANY.

HIGH-FREQUENCY FURNACE CIRCUIT.

Application filled February 13, 1926, Serial-No. 88,015, andiin GermanyFebruary 18, 1927.

This invention relates to high-frequency furnaces, but it may beunderstood that the invention is not limited to this special type `ofhigh-frequency loads, but may be applied I to' other types of loadsystems with which a high-frequency machine is to be connected.

It is well known to generate heat in electric conducting materials, inexposing them to the influence of the magnetic field of a.

surrounding coil carrying a high-frequency current. The'material knownas the resistor may thereby be regarded as the secondary of atransformer. The power transformed into heat is proportional to thesquare of the induced and consequently also of the inducing current.Moreover, the current in the secondary increases according to the numberof turns surrounding the furnace crucible supposingthe current in theprimary to be at constant value.

Therefore a great number of .primary turns is necessarily to be lookedfor. The

high-frequency current may be obtained by arc or spark discharges, ormaydirectly be supplied by a high-frequency alternator, for which lattermode of supply the connections given by the invention have a specialpractical importance. l

However there are great disadvantages in supplying. the 'furnace coildirectly by the generator. As the coil of the furnace, because of itsself-inductance, presentsa considerable resistance to the high-frequencycurrents, it is unfavorable to connect the furnace coil directly with'the terminals of the high-frequency generator. By connecting a variablecondenser in the known manner in parallel relationship to .the furnacecoil and tuning the circuit thus obtained to the .alternator frequency,the alternator may bei freed from wattless currents and its efiiciencyimproved, this connection being known under the 'term of parallelresonance. Further it is well known that the efficiency of an electricsystem has its greatest value, if 'theinner resistance of the generatoris equal to the resistanceof the outerload circuit. This condition isobviously in the most cases not to be realized in connecting a givenalternator with a given load according to the aforesaid manner.

Another method of connecting a high-frequency alternator with theheatingcoil of a high-frequency furnace hitherto known consists ininserting a condenser eventually in connection with a further tuningcoil in series with the coil of 'the furnace, or in other words inemploying a series resonance connection instead of the above mentionedparallel resonance connection. The advantage of Athis arrangement liesin that'the generator may be designed for nornial voltages and yet thatthe number of the furnace turns and consequently the efficiency of thelatter may bev increased. If the nat-` ural frequency of this circuit istuned to the frequency of the current of the high-frequency generator,the working of the generator may be considerably improved. As hasalready been set forth 'the furnace coil requires a great number ofampere-turns i. e. very strong currents if the resistor is to be heatedto a. very high temperature; when tuning to resonance, however, thevalue of the cos p obtained by'this connection is a very low one due tothe high wattless currents which also pass through the generatorproducing considerable losses. The generator therefore, has to bedesigned with respect to the Whole power output ,(apparent power)and'its size will be a multiple of the size corresponding to its purewattless power output.

It is the object of the present invention to overcome .these`disadvantages inherent to both a pure parallel resonance connection anda series resonance connection. The invention consists in :that thegenerator isV coupled to a part of a timing means of a circuitcontaining the load impedance of the furnace coil and additional tuningmeans and being tuned to the generator frequency,` so that in otherwords on the one hand the Iwattless currents of the furnace coil arecompensated and on the other hand the inner resistance of the alternatormay be adapted forV the resistance of the load resistance by means oftransformation.v Most favorablya direct coupling is employed which maybe made variable by means of tappings, this presenting the possibilityto the alternator to supply only the pure/.watt power for the resonantcircuit containing the load impedance of the heating coil. But aninductive or capacity coupling will do as well. In all ofv the cases avery great amplitude of the currents in the furnace coil is attained, ifthe circuit containing this coil is at least approximately resonant tothe generator frel quency and the alternator is counter-connected onlyto a part of the voltage drop in this resonant circuit and at the sametime has to supply only the Watt power dissipated in the resonantcircuit, and may be designed for normal values.

In order that the invention may be best understood reference should betaken to the accompanying drawings of Which Fig. 1 represents the Wellknown series resonance con-` nection, and Figures 2, 3 and 4 connectionsaccording to the invention, Fig. 2 employing a direct coupling of thegenerator to a part of the circuit resonant to the generator frequencyand consisting of the load impedance and additional timing means,Whereas Figures 3 and f4 show a corresponding inductive and capacitycoupling. The re erences in the single figures corresponding to eachother refer to like elements.- H is a high-frequency alternator, whichmay be of the usual variable inductancev type, .F represents the heatingcoil of a high-frequency furnace, but which may be replaced by someother high-frequency load impedance, S represents an additional tuningself-induction coil and C a capacity placed in series with the furnacecoil. Referring to Figure 1, the circuit H C S F is tuned to thealternator frequency and presents the disadvantages above referred to.According to Figure 2, the alternator is coupled directly with the partS, of a selfinductance forming part of the resonant circuit S, SCF,Which part may be made variable by 'means of tappings for the reason tobe described as follows. In Figure 3 the coupling is an inductive one bymeans of the transformer T andin Figure 4 there is employed a capacitycoupling by means of capacity C1. In Fig. 2 the circuit S, SCF, inFigure 3 the circuit FCS and secondary of T, and in Figure'4 the circuitFCS@1 are tuned to the frequency 'of the `high-frequency alternator H.

Further by suitably dimensioning the transformation ratio of thealternator to the resonant circuit e. the tapping at coil S1 in Fig. 2,or the'ratio of primary and secondaryturns of the transformer 'l in Fig.3 and the Value of the capacity C, in Fig. 4, it may be obtained thatthe alternator Works at its highest -efliciency and is only loaded WithWattless currents. This is apparently the case, When the terminalvoltage of the alternator having a fixed Value by its excitation and itsnumber of revolutions is equal to thevoltage drop at the terminals ofthe part of the resonant circuit to Which it is counterconnecte-d andmay also be explained in that the circuitcontainingrthe load coilcarries -very high resonance currents, very high at least With respectto the current flowing through the alternator and the alternator isWorking upon this circuit like upon a supply line fed by otherconsiderably stronger alternators, and that this terminal voltage havinga constant fixed value it is forced to supply a leading or a lagging ora pure Wattless current 'according lo the voltage of the resonantcircuit it is oiiiitercon nected to, the potentials at thel alternatorterminals and the coupling element of the resonant circuit beingnecessarily at equal value.

A high-frequency furnace inclu-ded into a A ing action Will thereforedecrease continually.

According to a further feature of the invention this disadvantage is'overcome by employing in the resonant circuit timing means to becontinuously varied during operationv such as variometers, variablecondensers eventually 'in connection with condensers and inductancesprovided With tappings, by means of which the variation of the impedanceof the furnace coil may continuously be balance-d in order to maintainthe original tuning conditions. But it may be seenthat the coupling ofthe alternator in this case must be regulated .too accordingly andaccording to this novel feature of the invention the circuit parallel tothe alternator must contain equally. continuously variably tuning means,or the transformation ratio of the alternator to the resonant circuit ismade' variable. Referring to Fig- Fure 2-'4 this latter condition may beob- .tained by substituting the self-inductance of Fig. 2 by a variableinductancc Varionieter, or by making the tapping continuously variable),or by making the coupling by transformer T Fig. 3 equally variable byuse of a vario-coupler or by employing a variable condenser C1 in thearrangement according to Fig. 4.

What we claim is n 1. ln current supplying means for high frequencymelting furnaces, a high frequency alternator machine, a load4impedance, a circuit including said impedance, said circuit having anatural frequency substantially resonant With the machine frequency,tuning means in said'circuit in series with the load impedance, andtiming means in said circuit in parallel with the load impedance andvari ably connected with the terminals of the alternator machine.

2. In current supplying means for high frequency melting furnaces, ahigh frequency alternator machine, a heating coil, a circuit includingsaid coil, said c'ircuit having a natural frequency substantiallyresonant with the machine frequency, `tuning means in said c1rcu1t 1nseries with the heatlng coll,-

and tuning means in said circuit in parallel with the coil and variablyconnected with thev terminals of the alternator machine.

8. In current supplying means for high frequency melting furnaces, ahigh frequency alternator machine, a load impedance, a circuit includingsaid impedance, said circuit having a natural frequency substantiallyresonant with the machine frequency, tuning means in said circuit inseries with the load lmpedance, and a varlable inductance 1n saldvI,circuit in parallel with the load impedance and connected with thelterminals of the alternator machine. y

4.' In a high-frequency current supply 'circuit for high-frequencymelting furnaces, a high-frequency dynamo machine, a heating coil 'tobeY supplied by said machine, a circuit Substantially resonant tothe'generator frequency and containing said heating-coil,

and a variablel inductance forming part of said resonant circuit andconnected to the terminals of the dynamo machine.

5. In a high-frequency feed circuit forhigh-frequency melting furnaces,a high-frequency dynamo machine, a heating coil to be supplied by .saidmachine, a circuit substantiall'yresonan't to the machine frequency andv containing said' heating coil, tuning means in said circuit, and aninductance forming part ofsaid resonant circuit and connected withtheterminals of the dynamol machine. 6. In `a high-frequency feedcircuit for high-frequency melting furnaces a highfrequency' dynamomachine, a heating coil to be sippliedby' said machine, a circuitlsubstantially resonantgto the machine frequency and containing saidheating coil, tuning means in said circuit, and coupling means betweensaid c1rcu1t and the dynamo machine, said means being in parallel withsaid circuit and connected to the terminals of the dynamo machine.- y

7. .In a high-,frequency feed circuit for high-frequency meltingfurnaces, a highfrequency dynamo machine, a load impedance to besupplied by said machine, a circuit substantially resonant to thefrequency of the' machine and containing said load impedance, variabletuning means in said cirsuit, and variable coupling means between salddynamomachine and said clrcuit.

8. In a high-frequency feed circuit forhigh-frequency melting furnaces,a highfrequency dynamo machine, a heatin coil to be supplied by saidmachine, a circuit substantially resonant to the machine frequency andcontaining said heating coil,

.tuning means in series in said circuit, aninductance shunted acrosssaid circuit, and Y means for .Variably connecting said inductance tothe terminalsof the dynamo machine.' l

to be supplied by said machine, a circuitsubstantially resonant to themachine frequency and containing said heating. coil, avariableinductance 'in said circuit, and a yvariable coupling betweensaid circuit and the dynamo machine.

. 11. In a high-frequency vfeed circuit for high-frequency meltingfurnaces, a highfrequency 'dynamo machine, a heating coil to be suppliedby said machine, a circuit substantially resonant -to the machinefrequency and containing said heating coil, variable tuning means insaidv circuit and in series' with said coils, and a variable parallelcoupling between said circuit and said dynamo machine. f

In testimony whereof we have aixed our signatures. V

' v DR. FELIX GERTH.

Dn. vVVILVHELM FISCHER.

coil

